This invention relates to a method and apparatus for controlling a continuously variable transmission for use with an automotive vehicle to change the engine brake force when the vehicle is coasting on a slope having a changing gradient.
Some automotive vehicles employ a continuously variable transmission having an input shaft coupled to the engine and an output shaft coupled to the drive shaft for transmitting a drive from the engine to the drive shaft. Such a continuously variable transmission operates with a speed ratio controlled in a manner to bring the speed of rotation of the input shaft into coincidence with a target value calculated as a function of engine throttle position (or accelerator pedal position) and vehicle speed. It is the current practice to decrease the target input shaft speed value as the throttle position decreases, If the vehicle is coasting on a downhill slope, the operator will release the accelerator pedal. This causes the throttle position to decrease so that the target input shaft speed value is changed (decreased) in a direction to weaken the engine brake. As a result, the operator would feel an excessive degree of vehicle acceleration in spite of the fact that the accelerator pedal is released and increase the frequency at which the operator depresses the brake pedal.
For example, Japanese Patent Kokai No. 6-81932 discloses a continuously variable transmission control apparatus intended to reduce the frequency at which the operator depresses the brake pedal when the vehicle is coasting on a downhill slope by setting a great lower limit for the target input shaft speed value to perform aggressive operate engine brake operations. With such a conventional apparatus, however, the target input shaft speed value changes frequently to provide a sense of incompatibility to the operator with changes in the gradient of the slope.
It is a main object of the invention to provide an improved continuously variable transmission control method and apparatus which can provide a smooth engine brake force change to meet the operators expectation therefor when the vehicle is coasting on a slope having a changing gradient with the accelerator pedal being released.
There is provided, in accordance with the invention, an apparatus for controlling a continuously variable transmission for use with an automotive vehicle including an accelerator pedal. The transmission has an input and output shaft. The transmission is operable at a variable speed ratio for transmitting a drive from the input shaft to the output shaft. The continuously variable transmission control apparatus comprises means for sensing vehicle operating conditions including vehicle acceleration, means for producing a released accelerator pedal indicative signal when the accelerator pedal is released, means for calculating a target value for the speed of rotation of the input shaft based on the sensed vehicle operating conditions, means for calculating a correction factor per predetermined unit time based on the sensed vehicle acceleration when the sensed vehicle acceleration exceeds a threshold value in the presence of the released accelerator pedal indicative signal, means for adding the correction factor to the target input shaft speed value to correct the target input shaft speed value at intervals of the predetermined unit time, and means for controlling the speed ratio to bring the input shaft speed into coincidence with the corrected target value.
In another aspect of the invention, the continuously variable transmission control apparatus comprises means for sensing vehicle operating conditions including vehicle deceleration, means for producing a released accelerator pedal indicative signal when the accelerator pedal is released, means for calculating a target value for the speed of rotation of the input shaft based on the sensed vehicle operating conditions, means for calculating a correction factor per predetermined unit time based on the sensed vehicle deceleration when the sensed vehicle deceleration exceeds a first threshold value in the presence of the released accelerator pedal indicative signal, means for subtracting the correction factor from the target input shaft speed value to decrease the target input shaft speed value at intervals of the predetermined unit time, and means for controlling the speed ratio to bring the input shaft speed into coincidence with the decreased target value.
In another aspect of the invention, there is provided a method of controlling a continuously variable transmission for use with an automotive vehicle including an internal combustion engine, an accelerator pedal and a drive shaft. The transmission has an input shaft coupled to the engine and an output shaft coupled to the drive shaft. The transmission is operable at a variable speed ratio for transmitting a drive from the engine to the drive shaft. The continuously variable transmission control method comprises the steps of sensing vehicle operating conditions including vehicle acceleration, producing a released accelerator pedal indicative signal when the accelerator pedal is released, calculating a target value for the speed of rotation of the input shaft based on the sensed vehicle operating conditions, calculating a correction factor based on the sensed vehicle acceleration when the sensed vehicle acceleration exceeds a threshold value in the presence of the released accelerator pedal indicative signal, adding the correction factor to the target input shaft speed value to correct the target input shaft speed value, controlling the speed ratio to bring the input shaft speed into coincidence with the corrected target value, and continuously repeating the above sequence of steps at uniform intervals of time to effect changes in the target input shaft speed value in response to changes in the vehicle acceleration.
In still another aspect of the invention, the continuously variable transmission control method comprises the steps of sensing vehicle operating conditions including vehicle deceleration, producing a released accelerator pedal indicative signal when the accelerator pedal is released, calculating a target value for the speed of rotation of the input shaft based on the sensed vehicle operating conditions, calculating a correction factor based on the sensed vehicle deceleration when the sensed vehicle acceleration exceeds a first threshold value in the presence of the released accelerator pedal indicative signal, subtracting the correction factor to the target input shaft speed value to decrease the target input shaft speed value, controlling the speed ratio to bring the input shaft speed into coincidence with the decreased target value, and continuously repeating the above sequence of steps at uniform intervals of time to effect changes in the target input shaft speed value in response to changes in the vehicle deceleration.